If you use the A14 4 speed, it'll have the same gearing as the A12. Interestingly the extra torque means you can use less throttle to get the same speed, so it seems easier. The only way is to find an overdrive 5 speed to suit the A15, which are quite rare over here in Australia. They are easier to come by in South Africa, being used in the Baky ute until they stopped making it. It does make a difference fitting the 5 speed, & you never know where they'll be. You can only look, I suppose.